The present invention relates to an image forming apparatus, such as a copier, facsimile, printer, and the like, and specifically relates to an image forming apparatus equipped with an optical writing apparatus that uses an LED.
A well known method that is applied to an image writing apparatus for a photoreceptor of an image forming apparatus uses an LED array. This method directs modulated light emitted from an LED array, which is arrayed in a highly dense state, onto a photoreceptor via a convergence rod lens array to form an image. Because LED light-emitting elements the number of which corresponds to the resolution are arrayed in a straight line or zigzag pattern thereby making up an optical scanning mechanism, this method makes it possible for an optical scanning system to be compact.
However, generally, the focal depth of an LED writing head (hereafter, referred to as LED head) that uses a convergence rod lens array is shallow (50 to 100 μm), and deviation of the focus position causes image quality to deteriorate. Accordingly, in an image forming apparatus with an LED head built in, it is necessary to prevent the distance between the LED head and the photoreceptor from deviating from a reference position. For example, if there is eccentricity of a drum-type photoreceptor or core deviation of a drum support mechanism, an electrostatic latent image formed on the photoreceptor becomes partially defocused, and when the image is developed by means of toner, in some cases, the image becomes blurred. Even if a solution for this kind of problem is sought in terms of trying to maintain work accuracy, the improvement of machine size tolerance and assembly accuracy are limited, and costs increase.
To solve this problem, Japanese Unexamined Patent Application Laid-Open No. 62-250466 (hereinafter Patent Document 1) discloses a method that changes the position of a light-emitting section and an image-forming element by detection of a photoreceptor's surface deviation. Furthermore, Japanese Unexamined Patent Application Laid-Open No. 04-246668 (hereinafter Patent Document 2) discloses a method in which a reference position is provided at each of both end portions of the LED head, and the distance between the reference position and the photoreceptor is measured by a deviation detecting sensor; the LED head slightly moves thereby positioning the LED head.
FIG. 8 shows the outline of a conventional image writing apparatus that uses an LED head. This conventional technique is an embodiment that has a configuration of an image writing apparatus of an LED printer described in Japanese Unexamined Patent Application Laid-Open No. 03-221471 (hereinafter Patent Document 3). This embodiment comprises a fixing member 301 which holds an LED head 3, a deviation detecting sensor 2 which detects a deviation of gap d between the drum-type photoreceptor 1 and the LED head 3, a solenoid 401 and a plunger 402; and further comprises a driving section 4 which moves the LED head 3 back and forth against the photoreceptor 1 by changing the plunger's 402 suction according to the change of the driving current to the solenoid 401, and a servo control circuit 5 which controls a driving section 4 by using the detected output of the deviation detecting sensor 2 as a deviation signal.
The above-mentioned conventional technique described in Patent Document 3 detects the deviation of gap d by the deviation detecting sensor 2 according to the circumferential surface deviation of the photoreceptor 1 and moves the LED head 3 back and forth, thereby keeping the focus position of the LED head 3 constant. However, the detection signal sent from the deviation detecting sensor 2 sometimes contains unnecessary noise or high-frequency components which the LED head's 3 driving section 4 cannot track. Accordingly, control of the apparatus according to such a signal may cause the servo control circuit 5 to malfunction due to noise or external vibration, or cause stability of the apparatus to deteriorate due to the occurrence of unnecessary vibration.
Generally, rotational frequency components of the photoreceptor drum mostly account for the photoreceptor's circumferential surface deviation signal due to its nature, and the value of an effective position control signal continuously and moderately changes. Therefore, high-frequency components which are greater than a prescribed frequency component do not have a significant direct influence on the focal depth of the LED head. Accordingly, high-frequency components are not necessary when controlling the position of the LED head. Furthermore, a deviation signal which is effective for the control of the photoreceptor drum surface can be a relatively low-frequency signal that moderately changes. This is to some extent presumable, and therefore, without controlling the position in real time by using a deviation detecting sensor, sufficient effects can be acquired by controlling the apparatus according to the measured results that have been beforehand obtained.